Re: [Qemu-devel] [RFC V4 01/30] qcow2: Add deduplication to the qcow2 specification.

[Eric Blake <eblake@redhat.com>]

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On 01/02/2013 09:16 AM, Benoît Canet wrote:
> Signed-off-by: Benoit Canet <benoit@irqsave.net>
> ---
>  docs/specs/qcow2.txt |  100 +++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 99 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index 36a559d..c9c0d47 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -80,7 +80,12 @@ in the description of a field.
>                                  tables to repair refcounts before accessing the
>                                  image.
>  
> -                    Bits 1-63:  Reserved (set to 0)
> +                    Bit 1:      Deduplication bit.  If this bit is set then
> +                                deduplication is used on this image.

This part seems fine; and I agree with making this an incompatible
feature (as an older qemu that does not understand dedup would not keep
the dedup table up-to-date).

> +                                L2 tables size 64KB is different from
> +                                cluster size 4KB.

Umm, doesn't the cluster_bits (bytes 20-23 of the header) determine the
size of a cluster, rather than assuming a cluster is always 4KB?  And
later on, the spec says that "L2 tables are exactly one cluster in
size.", so I'm not sure what this comment is doing here.  Or are you
stating that deduplication _also_ has an L2 table, which is fixed in
size (unlike the normal L2 table for actual data)?

> +== Deduplication ==
> +
> +The deduplication extension contains the informations concerning the

s/informations concerning the/information concerning/

> +deduplication.
> +
> +    Byte   0 - 7:   Offset of the RAM deduplication table
> +
> +          8 - 11:   Size of the RAM deduplication table = number of L1 64-bit
> +                    pointers
> +
> +              12:   Hash algo enum field
> +                        0: SHA-256
> +                        1: SHA3
> +                        2: SKEIN-256
> +
> +              13:   Dedup stategies bitmap

s/stategies/strategies/

> +                        0: RAM based hash lookup
> +                        1: Disk based hash lookup
> +
> +Disk based lookup structure will be described in a future QCOW2 specification.

Does that mean that strategy must be 0 for now?

> +
> +== Deduplication table (RAM method) ==
> +
> +The deduplication table maps a physical offset to a data hash and
> +logical offset. It is used to store permanently the informations required to

s/store permanently the informations/permanently store the information/

> +do the deduplication. It is loaded at startup into a RAM based representation
> +used to do the lookups.
> +
> +The deduplication table contains 64-bit offsets to the level 2 deduplication
> +table blocks.
> +Each entry of these blocks contains a 32-byte SHA256 hash followed by the
> +64-bit logical offset of the first encountered cluster having this hash.
> +
> +== Deduplication table schematic (RAM method) ==
> +
> +0       l1_dedup_index                                              Size
> +              |
> +|--------------------------------------------------------------------|
> +|             |                                                      |
> +|             |        L1 Deduplication table                        |
> +|             |                                                      |
> +|--------------------------------------------------------------------|
> +              |
> +              |
> +              |
> +0             |           l2_dedup_block_entries
> +              |
> +|---------------------------------|
> +|                                 |
> +|    L2 deduplication block       |
> +|                                 |
> +|                 l2_dedup_index  |
> +|---------------------------------|
> +                         |
> +         0               |              40
> +                         |
> +         |-------------------------------|
> +         |                               |
> +         |    Deduplication table entry  |
> +         |                               |
> +         |-------------------------------|
> +
> +
> +== Deduplication table entry description (RAM method) ==
> +
> +Each L2 deduplication table entry has the following structure:
> +
> +    Byte  0 - 31:   hash of data cluster
> +
> +         32 - 39:   Logical offset of first encountered block having
> +                    this hash
> +
> +== Deduplication table arithmetics (RAM method) ==
> +
> +Entries in the deduplication table are ordered by physical cluster index.
> +
> +The number of entries in an l2 deduplication table block is :
> +l2_dedup_block_entries = dedup_block_size / (32 + 8)

I'd write this as CEIL(dedup_block_size / (32 + 8)) to make it clear
that it rounds up...

> +
> +The index in the level 1 deduplication table is :
> +l1_dedup_index = physical_cluster_index / l2_block_cluster_entries
> +
> +The index in the level 2 deduplication table is:
> +l2_dedup_index = physical_cluster_index % l2_block_cluster_entries
> +
> +cluster_size = 4096
> +dedup_block_size = 65536
> +l2_size = 65536
> +
> +The 16 remaining bytes in each l2 deduplication blocks are set to zero and
> +reserved for a future usage.

...and move this paragraph closer to the point where you mention the
rounding up in size.

> +
>  == Host cluster management ==
>  
>  qcow2 manages the allocation of host clusters by maintaining a reference count
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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